This isn't very relevant anymore but back in the bad old days of dial-up modems, the name of the ISP game was to get as many customers as one could, loading up the system to the point where it was nearly but not quite falling over. At that point you maximise your profits. You will also have roughly the same number of customers leaving as you will be getting new ones, but that's OK provided the system is fully loaded. Customers are irrelevant to the big picture.
Pipex was one of the most notorious players in that game.
Broadband is different, in that an ISP has no modem banks** and phone lines; indeed has no physical existence other than a building somewhere, with a fibre connection into BT's ATM network, and from there on it all goes via BT. The only customer presence an ISP needs is the support line, and that is what you pay for (or pay a bit less for, if it is in India). A lot of ISPs are also evidently run by what I call crooks (like Pipex and many others were back then) - IMHO only a crook will subcontract support to a monkey call centre and charge a premium rate for calling it.
The actual technology for creating a reliable ISP is no rocket science and is well known. It just costs money to implement properly.
At current prices, anything less than say £18/month is likely to be a bunch of crooks, will give you hassle, and won't last. I don't smoke but the difference between a good ISP and a crap one is roughly one fag packet per month.
(** the better ISPs do have a deal with some network outfit to provide a dial-in number, for modem and mobile users)